The invention relates generally to measurement apparatus and methods and, more particularly, to apparatus and methods for performing spatial measurements of energy beams.
With increasing use of information display devices such as color cathode ray tubes (CRTs) in information processing, computer aided design/computer aided manufacturing (CAD/CAM), and graphic display applications including air traffic control, as well as in the traditional field of home entertainment, demand is growing for efficient high-accuracy optical measurement systems and techniques to aid in manufacturing and servicing of information display devices. It is particularly desired to provide a more efficient technique for accurately measuring the convergence of a color CRT.
Color CRTs typically employ three beams of electrons conveying intensity information for red, green, and blue portions of a color CRT display. The beams are focused on tiny triads of phosphor elements, with each separate element of the triad being a separate type of phosphor element respectively emitting red, green, and blue light when activated by incident electron beams conveying respective red, green, and blue intensity information. The triads may be arranged in patterns such as tiny triangles of round phosphor elements or groups of longitudinally adjacent stripes. Many CRTs employ a structure such as a shadow mask or grille to ensure that the red beam will only strike the red phosphor elements, the green beam will only strike the green phosphor elements, and the blue beam will only strike the blue phosphor elements.
Such structure is shown in FIG. 1, which is a schematic representation of a portion of a color CRT 10. CRT 10 includes an evacuated glass envelope 12 including electron guns 14, 16, and 18 which respectively generate beams 20, 22, and 24 of electrons, the intensity of which represents the intensity of respective red, green, and blue information to be displayed on the CRT. This intensity data is supplied to guns 14, 16, and 18 by associated circuitry (not shown). In other types of CRTs, beams 20, 22, and 24 may be generated by a single gun.
Beams 20, 22, and 24 are directed by deflection apparatus represented schematically at 26 through a shadow mask 28 to patterns of phosphor elements 30 adhered to the inside of the face 32 of CRT 10. Adjustment circuitry 34 is provided to adjust deflection apparatus 26 to ensure that beams 20, 22, and 26 converge to the same area on tube face 32, as shown in FIG. 1. If adjustment is not properly performed, the beams will not activate the same triads of phosphor elements 30, resulting in color fringeing of the image displayed in CRT 10. This misconvergence is shown schematically in FIG. 2, wherein beams 20, 22, and 24 respectively are incident upon areas 36, 38, and 40 of tube face 32 which do not coincide.
A schematic diagram of prior art apparatus for measuring the convergence adjustment of a CRT is shown in FIG. 3. A monitor 50 containing a CRT to be tested is supplied with a signal from a generator 52 to produce a pattern, such as vertical white line 54. Light from line 54 travels over an optical path 57 such that an image of white line 54 is generated by optical apparatus 58 at an image scanning plane 56. The image is transmitted by an optical system 60 to a detector apparatus 59. Detector apparatus 59 includes variable filter 62, a shutter 63, and a detector device such as a photomultiplier tube (PMT) 64 which provides an output signal representative of the intensity of radiation incident upon a target surface within the detector device. PMT 64 of detector apparatus 59 is connected to a control apparatus 66, which includes an amplifier 68, the output of which is supplied through an analog-to-digital converter (ADC) 70 to a microcomputer 72. Control apparatus 66 is connected to a host computer 67.
A slit aperture 74 is positioned in optical path 57 at image scanning plane 56. FIG. 4 shows a top view of aperture 74, viewed in the direction of arrow 81 of FIG. 1.
Microcomputer 72 is connected to an output controller 73 which operates shutter 63 to assist in drift compensation. Microcomputer 72 also connected through controller 73 to variable filter 62 to permit control of the spectral transmission characteristics of filter 62. Microcomputer 72 is also connected through a digital-to-analog converter 79 to deflection circuitry 80 of monitor 50 to permit control of white line 54 generated by monitor 50.
To perform a convergence measurement using the prior art apparatus of FIG. 3, a signal was transmitted from microcomputer 72 to deflection circuitry 80 to cause line 54 to scan horizontally across the face of monitor 50 in small increments of, for example, 1/10 of the width of line 54. Light from excited red, green, and blue phosphor elements thus passed through slit aperture 74 to PMT 64, with filter 62 initially adjusted for a wave length of, for example, 630 nanometers, to pass light emitted by the red phosphor. Scanning of line 54 across the face of monitor 50 allowed PMT 64 to generate an intensity versus displacement profile of the electron beam that excited the red phosphors of monitor 50.
Microprocessor 72 then supplied signals to deflection circuitry 80 to cause line 54 to return to its original position. Filter 62 was then adjusted to a setting of, for example, 540 nanometers, to pass light emitted by the green phosphor. Signals were then supplied to deflection circuitry 80 to cause line 54 to be scanned in 1/10 beam width increments across the face of monitor 50, thus generating an intensity versus displacement profile of the electron beam that excited the green phosphor. Filter 62 was then adjusted to a frequency of, for example, 450 nanometers, to pass light emitted by the blue phosphor and line 54 was similarly scanned to provide an intensity versus displacement profile of the electron beam which excited the blue phosphor. The output from apparatus 66 to host computer 67 thus consisted of horizontal intensity profiles for each of the three electron beams. Software in host computer 67 then computed both the X coordinate of the centroid, that is, the center of energy, for each beam, as well as the spatial relationship in millimeters between the centroids of each of the three beams. These intensity profiles were free of variations of data caused by the shadow mask structure, since aperture 74 was not scanned relative to the phosphor/shadow mask structure. The profiles thus generated also included any interaction among the three beams, since all three beams were activated during measurement. It is generally accepted that the intensity profile of an electron beam incident upon a phosphor coated surface can theoretically be described by a Gaussian function. The resultant intensity profiles obtained by the method of FIG. 3 accordingly exhibited approximately Gaussian distribution.
The above described method of scanning electron beams past aperture 74 gives good electron intensity profile data which is relatively free from undesired effects caused by phosphor graininess or noise, thus facilitating the centroid determination. It is therefore satisfactory in many applications. However, it is not always possible to provide a connection between control apparatus 66 and the internal circuitry 80 of the monitor under test. An alternative method is to scan aperture 74 relative to a stationary image of line 54. However, such method using prior art apparatus produced intensity variations caused by the shadow mask structure. Determination of a true intensity profile from such data, and hence the centroid location, is extremely difficult and susceptible to error. An additional problem of the above described method is the difficulty in accumulating data representing 100% of the image energy, a problem caused by overlapping or non-continuous slit measurements and by the interaction between slit width and aperture position.
In order to obtain highest accuracy, it was therefore necessary to compensate for the effect of the measuring slit width, due to interactions between slit width, line width, and line deflection increments. An example of a spatial profile obtained in this manner is shown in FIG. 5. The X axis of FIG. 5 represents horizontal distance across the width of vertical line 54, and the Y axis represents intensity. FIG. 5 includes three curves 82, 84, and 86 respectively representing the spatial intensity profiles of red, green, and blue components of white line 54.
The centroid of each beam was calculated by summing the intensity values for all line deflection increments to determine the total area under the measured intensity profile curve. That is, for each position in the X direction, there was a corresponding measured intensity value. For each position in the X direction, a summation value was calculated equal to the sum of the intensity values for all preceding positions. These summation values were stored in the host computer as a function of the X position and the fifty percent value calculated. The X positions of summation values on either side of the fifty percent value were determined, and the X position of the fifty percent value calculated by interpolation. This interpolated X position of the fifty percent summation value was the centroid of the beam.
However, errors were introducted into the centroid calculation due to noise present on curves 82, 84, and 86, the effects of which are extremely difficult to compensate for. Another difficulty in obtaining convergence measurements is the effect of CRT line jitter on data obtained through slit aperture 74.